Post-AGB circumstellar envelopes undergo drastic changes due
 to the combined action of powerful stellar winds in the proto-PN
 phase and intense stellar radiation (UV to X-ray) in the PN phase.
 These effects produce extreme physical conditions in the neutral
 gas (high densities, temperatures, and velocities) through shocks
 and photon dominated regions (PDRs), which are now realized to be
 key features of PN formation. We propose a systematic study of
 these extreme regions in a selected sequence of proto-PNe and PNe.
 Measurements will be made of the principal diagnostic atomic lines
 of the excited neutral gas in envelopes at different stages of
 evolution. In specially targeted cases we plan to obtain the
 spatial distribution of the emission, the kinematics of the gas,
 and (through observations of the ground vibrational state of
 molecular hydrogen lines) the physical conditions of the molecular
 gas. The above project is thought to complement the Central Program
 which does not include studies of the spatial extension of evolved
 planetary nebulae or high spectral resolution measurements of the
 main infrared cooling lines. The proposed observations will supply
 fundamental data on the highly excited gas at different stages of
 envelope evolution, and will provide new insights into the extreme
 physical processes which play an important role in PN formation.